Valve assemblies of the type employing a shiftable spool valve commonly employ a mechanical spring for returning the valve spool to its normal end position. However, as the pressure of the fluid controlled by the valve increases, the valve seals are increasingly deformed, so that a larger return force is required for returning the valve to its normal position. For this reason, such valve assemblies conventionally use a fluid-assisted spring for returning the valve spool to its normal position, whereby the mechanical spring force is only strong enough to return the spool at zero or very low fluid pressure, whereas the air spring provides additional return force as the fluid pressure increases. These valve assemblies are normally provided with an actuator associated with one end of the valve spool for causing axial displacement thereof from one end position into an opposite end position. This actuator often comprises a fluid pilot but may be of many different types, such as manual, mechanical or electrical. The fluid-assisted spring is normally associated with the end of the valve spool opposite the actuator, and conventionally comprises a closed chamber defined between the valve spool and the housing for confining therein a mechanical spring. This chamber is normally in continuous communication with the inlet port, and for this purpose a passageway is normally provided either internally or externally of the housing. This makes manufacture of the housing more complex or, in the alternative, makes the overall housing bulky and cumbersome. This also results in the housing for a valve assembly employing a fluid-assisted spring being different from the housing for a valve assembly which does not employ a fluid spring, thereby preventing standardization of the housings.
It is thus an object of the present invention to provide an improved valve assembly capable of being provided with a fluid-assisted spring, which valve assembly overcomes the above-mentioned disadvantages.
More specifically, it is an object of this invention to provide an improved fluid-assisted spring means for use with a shiftable spool-type valve assembly, which spring means utilizes a communication passage extending interiorly of the valve spool for providing communication between the inlet port and the spring chamber, whereby the valve housing can be simplified and standardized.
Another object is to provide an improved valve assembly, as aforesaid, which utilizes an O-ring positioned within an annular groove on the valve spool, which annular groove is of substantially greater axial width than the diameter of the O-ring so that when the latter is shifted axially across the inlet port during shifting of the spool, the O-ring is axially shifted relative to the spool, whereby the O-ring displacement is greater than the valve spool displacement so that minimum shifting of the valve spool is possible.
A further object is to provide an improved valve assembly, as aforesaid, wherein the communication passage between the inlet port and the spring chamber extends axially from the spring chamber through the valve spool and terminates at the bottom of said groove, whereby the O-ring is rapidly shifted axially across the open end of the communication passage so that the latter provides substantially continuous communication between the inlet port and the spring chamber.
Other objects and purposes of the invention will be apparent to persons familiar with valve assemblies of this general type upon reading the following specification and inspecting the accompanying drawings.